Arsenic-free glasses

ABSTRACT

This invention is related to glasses for use as substrates in flat panel display devices, more specifically to a family of aluminosilicate glasses in which less than 0.2 mole percent and preferably no As 2  O 3  is used as a fining agent and the β-OH of the glass is maintained below about 0.5 mm.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional application Ser.No. 60/022,193, filed Jul. 19, 1996.

FIELD OF THE INVENTION

This invention is directed to arsenic-free glass compositions andmethods of making such glasses which are suitable for use as substratesin flat panel display devices without having to use arsenic containingmaterials.

BACKGROUND OF THE INVENTION

Liquid crystal displays (LCDs) are passive flat panel displays whichdepend upon external sources of light for illumination. They aremanufactured as segmented displays or in one of two basicconfigurations. The substrate needs (other than being transparent andcapable of withstanding the chemical conditions to which it is exposedduring display processing) of the two matrix types vary. The first typeis intrinsic matrix addressed, relying upon the threshold properties ofthe liquid crystal material. The second is extrinsic matrix or activematrix (AM) addressed, in which an array of diodes,metal-insulator-metal (MIM) devices, or thin film transistors (TFTs)supplies an electronic switch to each pixel. In both cases, two sheetsof glass form the structure of the display. The separation between thetwo sheets is the critical gap dimension, of the order of 5-10 μm.

Intrinsically addressed LCDs are fabricated using metal depositiontechniques, typically at temperatures ≦350° C., followed by standardmetal etching procedures. As a result, the substrate requirementstherefor are often the same as those for segmented displays.Soda-lime-silica glass with a barrier layer has proven to be adequatefor most needs. A high performance version of intrinsically addressedLCDs, the super twisted nematic (STN) type, has an added requirement ofextremely precise flatness for the purpose of holding the gap dimensionsuniform. Because of that requirement, soda-lime-silica glass made usingthe float glass manufacturing process must be polished. Such polishingprocesses are expensive and time consuming, and generate a large amountof glass particles which have the potential to negatively impact furtherprocessing of the glass sheets. Alternatively, glass can be formed usinga process which does not require polishing, e.g. fusion downdraw.

Extrinsically addressed LCD's can be further subdivided depending uponthe nature of the electrical switch located at each optical element(subpixel). Two of most popular types of extrinsically (or activematrix, AMLCD) addressed LCD's are those based on either amorphous(a-Si) or polycrystalline (poly-Si) silicon thin film transistors(TFT's).

U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) lists four desirable propertiesfor a glass to exhibit in order to fully satisfy the needs of asubstrate for extrinsically addressed LCD's:

First, the glass must be essentially free of intentionally added alkalimetal oxide to avoid the possibility of alkali metal contamination ofthe TFT;

Second, the glass substrate must be sufficiently chemically durable towithstand the reagents used during the manufacture of the TFT;

Third, the expansion mismatch between the glass and the silicon presentin the TFT array must be maintained at a relatively low level even asprocessing temperatures for the substrates increase; and

Fourth, the glass must be capable of being produced in high quality thinsheet form at low cost; that is, it must not require extensive grindingand polishing to secure the necessary surface finish.

That last requirement is most difficult to achieve inasmuch as itdemands a sheet glass production process capable of producingessentially finished glass sheet. A process capable of meeting thisrequirement is a particular downdraw process known as the overflowdowndraw, or fusion, sheet manufacturing process. The overflow downdrawprocess is described, for example, in U.S. Pat. No. 3,338,696 (Dockerty)and U.S. Pat. No. 3,682,609 (Dockerty). Fusion formed glass sheets,unlike float glass sheets, are sufficiently flat that they do not needto be polished after forming. Two glasses which meet the aboverequirements, Corning Incorporated Codes 7059 and 1737 sheet glass, arecurrently employed as substrates for extrinsically addressed LCD's.These glasses are made using the overflow downdraw process, and hence donot require polishing after forming.

Recent improvements in the resolution of extrinsically addressed LCD'shave led to the development of a fifth glass requirement, that is, ahigh glass strain point. This property is used as an indication of thethermal shrinkage of the glass. As can be appreciated, the lower thestrain point, the greater is this thermal shrinkage. Low thermalshrinkage is desirable for precise alignment during successivephotolithographic and other patterning steps during the TFT processing.Consequently, glasses having higher strain points are generallypreferred for extrinsically addressed LCD's, particularly those whichemploy poly-Si TFT technology. Thus, there has been considerableresearch to develop glasses demonstrating high strain points so thatthermal shrinkage is minimized during device processing. Corning Code1737 glass, which has the highest strain point (666° C.) in the AMLCDsubstrate industry, is rapidly becoming an industry standard. Concurrentwith their high strain points, these glasses often have high meltingtemperatures, e.g. on the order of 1550°-1650° C.

Another technology termed "chip-on-glass" (COG) has further emphasizedthe need for the substrate glass to closely match silicon in thermalexpansion. Thus, the initial LCD devices did not have their driver chipsmounted on the substrate glass. Instead, the silicon chips were mountedremotely and were connected to the LCD substrate circuitry withcompliant or flexible wiring. As LCD device technology improved and asthe devices became larger and required finer resolutions, these flexiblemountings became unacceptable, both because of cost and of uncertainreliability. This situation led to Tape Automatic Bonding (TAB) of thesilicon chips. In that process the silicon chips and electricalconnections to the chips were mounted on a carrier tape, thatsubassembly was mounted directly on the LCD substrate, and thereafterthe connection to the LCD circuitry was completed. TAB decreased costwhile improving reliability and increasing the permitted density of theconductors to a pitch of approximately 200 μm--all significant factors.COG, however, provides further improvement over TAB with respect tothose three factors. Hence, as the size and quality requirements of LCDdevices increase, COG is demanded for those devices dependent upon theuse of integrated circuit silicon chips. For that reason, the substrateglass preferably demonstrate a linear coefficient of thermal expansionclosely matching that of silicon; i.e., a linear coefficient of thermalexpansion (0°-300° C.) between about 32-46×10⁻⁷ /°C., most preferably32-40×10⁻⁷ /°C.

Many of the glasses manufactured for flat panel display applications,particularly those which are formed by downdraw processes (e.g., thefusion or slot draw processes), are melted or formed using manufacturingequipment comprised of refractory metals, e.g. platinum or platinumalloys, particularly in the fining and conditioning sections of theprocess, where refractory metals are employed in order to minimize thecreation of compositional in homogenieties and gaseous inclusions causedby contact of the glass with oxide refractory materials. In addition,many of these manufacturing processes employ arsenic as a fining agent.This is because arsenic is among the highest temperature fining agentsknown, meaning that, when added to the molten glass bath, it allows forO₂ release from the glass melt even at high melting temperatures (e.g.above 1450° C.). This high temperature O₂ release, (which aids in theremoval of gases during the melting and fining stages of glassproduction), coupled with a strong tendency for O₂ absorption at lowerconditioning temperatures, (which aids in the collapse of any residualgaseous inclusions in the glass), results in a glass product essentiallyfree of gaseous inclusions. In addition, the oxidizing nature of thearsenic fining package allows for protection of the platinum based metalsystems by preventing contamination as a result of tramp metalsreduction. Other fining agents typically melt and release their oxygenfar too early when added as fining agents to high melting temperatureglasses and reabsorb O₂ too late during the conditioning process,thereby disabling their fining abilities. From an environmental point ofview, it would be desirable to find alternative methods of making suchhigh melting point and strain point glasses without having to employarsenic as a fining agent. It would be particularly desirable to findmethods for making such glasses via downdraw (especially fusion-like)processes. Unfortunately, previous efforts at doing so have beenhindered by the production of unacceptable amounts of bubbles in theglass. This has been a particular problem with glasses which employrefractory metals such as platinum or platinum containing alloys intheir molten glass delivery systems. This is because platinum can causean electrochemical reaction to occur with the glass which results inbubble formation on or near the platinum (commonly referred to asblistering) contacting region of the glass.

SUMMARY OF THE INVENTION

We have found that by maintaining a low amount of water in the glassduring the glass forming process, other fining constituents which arenormally less efficient at high melting temperatures (meltingtemperature is defined herein as the temperature at which the glassexhibits a viscosity of 200 poise), e.g. Sb₂ O₃, CeO₂, SnO₂, Fe₂ O₃, andmixtures thereof, can be employed if needed in place of As₂ O₃ tofacilitate successful fining of the glass. Maintaining a low amount ofwater in the glass thus enables the formation of high melting pointglasses (ie., glasses wherein the temperature at which the viscositycorresponds to 200 poise is greater than about 1500° C.) which areessentially or substantially arsenic-free. By substantially arsenic-freeit is meant that such glasses have less than 0.02 mole percent As₂ O₃(such amounts are normally present as a result of raw materialimpurity). The invention also enables the formation of such high meltingpoint glasses using manufacturing systems which employ platinum orplatinum containing alloys which contact the glass during the melting orforming steps of the manufacturing process. These methods areparticularly suited for forming glasses which are formed using adowndraw process, such as, for example, Corning Code 1737.

One manner of measuring the water content in the glass is by measuringbeta-OH (β-OH). β-OH, as used herein, is a measure of the hydroxylcontent in the glass as measured by IR spectroscopy, and is determinedusing the fundamental hydroxyl absorption, which for this materialoccurs at about 2809 nm. The β-OH is the linear absorption coefficient(absorbance/mm thickness) of the material at 2809 nm. The equation belowshows how β-OH is calculated from the sample's IR transmittancespectrum.

    β-OH=(1/X) LOG.sub.10 (T.sub.1 /T.sub.2)

where X is the sample thickness in millimeters, T₁ is the sampletransmittance at the reference wavelength (2600 nm) and T₂ is theminimum sample transmittance of the hydroxyl absorption wavelength (2809nm). The reference wavelength compensates for signal loss due to surfacereflections, scatter, and refraction in the sample, and is chosen from aregion of no absorption and as close as possible to the absorptionwavelength of interest.

In a preferred embodiment of the present invention for forming lowarsenic containing glasses via a downdraw sheet forming process, thebatch constituents are selected so that the resultant glass has a watercontent therein, as indicated by β-OH level, which is less than 0.5,more preferably less than 0.4, and most preferably less than 0.35.

Preferably, the glasses formed in accordance with the invention areformed using less than 0.1 mole percent As₂ O₃, and most preferably areessentially free of As₂ O₃ in the resultant glass. We have found thatsilicate glasses (especially aluminosilicate and borosilicate glasses)formulated to result in such β-OH values in the resultant formed glasscan be fined using less than 0.02 mole percent As₂ O₃, expressed as theamount of As₂ O₃ present in the resultant glass. Even when formed usinga downdraw process employing a platinum based metal delivery system,such glasses can be formed without any significant amounts ofelectrochemical blistering occurring. In the most preferred embodiment,in order to facilitate fining of these glasses, Sb₂ O₃, CeO₂, SnO₂, Fe₂O₃, and mixtures thereof are added to such glasses, alone or incombination, in an amount between about 0.02-2 mole percent. In apreferred embodiment, Sb₂ O₃ is added in an amount between about 0.2-0.5mole percent.

The water content or β-OH value of the glass can be reduced in a varietyof ways. For example, simply by appropriate selection of batchmaterials, the water level in the glass can be adjusted to some extent.Further water reduction can be achieved by adding drying agents, such ashalide materials. For example, halide containing materials may be addedin an amount which results in the final glass having a compositionbetween about 0.1 to 4 mole percent halide, more preferably 0.1 to 2mole percent halide, and most preferably about 0.1 percent halide. In apreferred embodiment for forming the glass composition disclosed in theexample, 0.4 mole percent chlorine is batched, e.g. as CaCl₂, resultingin about 0.15 to 0.19 mole percent Cl in the resultant glass.

Additionally, it is desirable to keep the sum of the partial pressuresof all dissolved volatile gases below 1 atmosphere. One method offacilitating this result is by limiting the amount of sulfur in theresultant glass by appropriate selection of batch materials. Preferably,selection of batch materials should be made so that the sulfur,expressed as SO₃, in the resultant formed glass is as low as possible,preferably less than 100 ppm, more preferably less than 50, and mostpreferably less than 25 ppm.

The methods in accordance with the present invention are particularlyadvantageous in forming high strain point aluminoborosilicate glassessuch as, for example, those having a composition, expressed in terms ofmole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                              60-73          MgO    0-5                                            Al.sub.2 O.sub.3                                                                        8-14          CaO    1-13                                           B.sub.2 O.sub.3                                                                         5-17          SrO    0-8                                            TiO.sub.2                                                                              0-5            BaO    0-14                                           Ta.sub.2 O.sub.5                                                                       0-5                                                                  ______________________________________                                    

More preferably, the base glass has a composition, expressed in terms ofmole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                             64-70     MgO             0-5                                         Al.sub.2 O.sub.3                                                                      9.5-14    CaO              3-13                                       B.sub.2 O.sub.3                                                                        5-12     SrO               0-5.5                                     TiO.sub.2                                                                             0-5       BaO             2-8                                         Ta.sub.2 O.sub.5                                                                      0-5       MgO + CaO + SrO + BaO                                                                          10-20.                                     ______________________________________                                    

With the exception of the low water content, glasses within thispreferred composition range are disclosed, for example, in U.S. Pat. No.5,374,595, the specification of which is hereby incorporated byreference. Preferred glasses formed in accordance with the presentinvention exhibit linear coefficients of thermal expansion over thetemperature range of 0°-300° C. between 32-46×10⁻⁷ /°C., more preferablybetween 32-40×10⁻⁷ /°C., strain points higher than 630° C., morepreferably higher than 640° C., and most preferably greater than 650° C.liquidus temperatures less than 1125° C., liquidus viscosities which aresufficient to enable formation by a downdraw manufacturing process,preferably greater than 400,000, and more preferably greater than600,000 poises (60,000 Pa·s), a weight loss of less than 2 mg/cm² afterimmersion for 24 hours in an aqueous 5% by weight HCl solution at 95°C., long term stability against devitrification at melting and formingtemperatures, and melting viscosities of about 200 poises (20 Pa·s) atless than 1675° C. The methods of the present invention may be employedin glasses having compositions within the boundaries set forth above,such as, for example, glasses listed as examples in U.S. Pat. No.5,374,595, thereby enabling such glasses to be fined and formed withouthaving to use arsenic.

In the most preferred glasses, in addition to the low water level, thelevel of Al₂ O₃ will exceed that of B₂ O₃ and in the most preferredglasses the composition will consist essentially, expressed in terms ofmole percent, of about

    ______________________________________                                        SiO.sub.2                                                                             65-69     MgO             1-5                                         Al.sub.2 O.sub.3                                                                      10-12     CaO             3-9                                         B.sub.2 O.sub.3                                                                        7-10     SrO             1-3                                         TiO.sub.2                                                                             0-3       BaO             2-5                                         Ta.sub.2 O.sub.5                                                                      0-3       MgO + CaO + SrO + BaO                                                                         11-16                                       ______________________________________                                    

Preferably, the ratio Al₂ O₃ :B₂ O₃ in such glasses is greater than 1.

The invention thus enables the formation of silicate sheet glasseshaving high melting points (greater than 1500° C.), as well as theformation of silicate sheet glasses using manufacturing processes whichemploy refractory metals such as molybdenum and platinum in theirforming regions. Forming region, as used herein, refers to the portionof the manufacturing process prior to which the final form of the glassis imparted to the glass, and includes the melting, conditioning, andfining portions of the manufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a humidity controlled enclosure for use in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to silicate glass compositions and methodsof making such silicate glass compositions while employing little or noarsenic. The preferred glasses are aluminosilicate or borosilicateglasses. The preferred manufacturing processes for such glasses is via adowndraw sheet manufacturing process. As used herein, downdraw sheetmanufacturing process refers to any form of glass sheet manufacturingprocess in which glass sheets are formed while traveling in a downwarddirection. In the fusion or overflow downdraw forming process, moltenglass flows into a trough, then overflows and runs down both sides of apipe, fusing together at what is known as the root (where the pipe endsand the two overflow portions of glass rejoin), and is drawn downwarduntil cool. The overflow downdraw sheet manufacturing process isdescribed, for example, in U.S. Pat. No. 3,338,696 (Dockerty) and U.S.Pat. No. 3,682,609 (Dockerty). One advantage to the fusion formingprocess is that the glass sheet can be formed without the glass surfacecontacting any refractory forming surfaces. This provides for a smooth,contaminant-free surface. In addition, this technique is capable offorming very flat and thin sheets to very high tolerances. Consequently,fusion formed glass sheets, unlike float glass sheets, do not requirecostly polishing steps for TFT and STN LCD applications.

Other forms of downdraw sheet forming techniques include the slot drawand redraw forming techniques. In the slot draw technique, molten glassflows into a trough having a machined slot in the bottom. The sheets ofglass are pulled down through the slot. The quality of the glass isobviously dependent on the accuracy of the machined slot. Redrawprocesses generally involve preforming a glass composition into a blockof some shape, then reheating and drawing the glass downwardly into athinner sheet product.

The glasses of the present invention preferably have less than 0.2 molepercent As₂ O₃, more preferably less than 0.1 mole percent As₂ O₃, andmost preferably less than 0.02 mole percent As₂ O₃ (an amount which isnormally present as a result of raw material impurity).

It is believed that the method described herein are applicable to a widevariety of glasses, particularly those formed via downdraw manufacturingprocesses which employ platinum in their forming regions. Application ofthe invention to Corning Code 1737 glass, for example, is demonstratedas follows, with reference to Table I below. These glasses were preparedin a laboratory-scaled continuous melting unit similar to the overflowdowndraw melting units typically used for commercial production of thistype of product. This experimental melting unit employs aplatinum/rhodium alloy refractory metal delivery system, wherein themolten glass contacts the platinum alloy metal. Example 4 of Table Icorresponds closely to commercially available Corning Code 1737 glass,and was fined accordingly using an amount of arsenic which resulted inabout 0.4 mole percent being present in the resultant glass. Examples 1,2, and 3 illustrate the effect that decreasing amounts of water has onthese compositions. As the β-OH values of the glass decrease, so do thegaseous inclusions (Inc./lb.) in glass. In these examples, gaseousinclusions are primarily a result of electrochemical blistering causedby the platinum alloy pipes which deliver the molten glass, andconsequently accurately mimic the manufacturing processes employingmetals such as platinum. Gaseous inclusions were measured on a per poundbasis over a period of two to three days. As illustrated by theexamples, the inclusions per pound dropped significantly with eachdecrease of β-OH value. The fact that this was done without having touse As₂ O₃ as a fining agent makes this accomplishment significant.

Table I records similar glass compositions of varying β-OH levels,expressed in terms of parts by weight on the oxide basis, illustratingthe invention. Inasmuch as the sum of the individual constituents totalsor very closely approximates 100, for all practical purposes thereported values may be deemed to represent weight percent. Table IArecords the same glass compositions expressed in terms of mole percenton the oxide basis. The actual batch ingredients may comprise anymaterials, either oxides or other compounds, which, when melted togetherwith the other batch components, will be converted into the desiredoxide in the proper proportions. For example, SrCO₃ and CaCO₃ canprovide the source of SrO and CaO, respectively. In Example 3, Cl wasadded as CaCl₂ at a level of 0.2 weight percent in excess of the batch,resulting in about 0.087 weight percent Cl retained in the resultantglass. About 2.7 weight percent water in excess of the batch was addedto Examples 1 and 4.

Table I also lists measurements of several chemical and physicalproperties determined on the glasses in accordance with techniquesconventional in the glass art. Thus, the linear coefficient of thermalexpansion (CTE) over the temperature range 0°-300° C. expressed in termsof ×10⁻⁷ /°C., and the softening point (S.P.), annealing point (A.P.),and strain point (St.P.) expressed in terms of °C., were determined byfiber elongation. The durability (HCl Dur.) in HCl was determined bymeasuring the weight loss (mg/cm²) after immersion in a bath of aqueous5% by weight HCl at 95° C. for 24 hours.

The liquidus temperatures (Liq.Temp.) of the glasses were measured usingthe standard liquidus method, which involves placing crushed glassparticles in a platinum boat, placing the boat in a furnace having aregion of gradient temperatures, heating the boat in an appropriatetemperature region for 24 hours, and determining by means of microscopicexamination the highest temperature at which crystals appear in theinterior of the glass. The melting temperature (M.P., in °C.) (definedas the temperature at which the glass melt demonstrates a viscosity of200 poises 20 Pa.s!) was calculated employing the Fulcher equation asfit to the high temperature viscosity data. The liquidus viscosity (Liq.Vis.) was also calculated using the Fulcher equation coefficients, andis expressed in terms of ×1,000,000 poises (100,000 Pa.s). SnO₂ wasadded to examples 1-3 in an amount suitable to replicate meltingconditions in production, wherein the tin electrodes employed in meltingthe glass result in residual tin oxide in the resultant glass.

                  TABLE I                                                         ______________________________________                                               1       2         3         4                                          ______________________________________                                        SiO.sub.2                                                                              59.49     58.82     58.91   57.07                                    Al.sub.2 O.sub.3                                                                       16.4      16.7      16.58   16.46                                    B.sub.2 O.sub.3                                                                        8.29      8.3       8.21    8.35                                     MgO      0.737     0.739     0.765   0.77                                     CaO      4.109     4.111     4.116   4.21                                     SrO      1.889     1.883     1.887   1.88                                     BaO      8.6       8.59      8.61    9.49                                     SnO.sub.2                                                                              0.062     0.09      0.092                                            Sb.sub.2 O.sub.3                                                                       1.857     1.852     1.856   0                                        As.sub.2 O.sub.3                                                                       0         0         0       1.11                                     % Added H.sub.2 O                                                                      2.70      0         0       2.7                                      Cl       0         0         0.087   0                                        β-OH                                                                              0.481     0.41      0.358   0.440                                    Inc./lb. 15.2      2.06      0.26    .21                                      S.P.     973       976       977     968                                      M.P.     1641      1638      1644    1625                                     St. Pt.  660       665       664     658                                      A.P.     717       719       720     714                                      Liq. Temp.                                                                             1080      1080      1090    1050                                     Lig. Vis.                                                                              1.37      1.4       1.06    2.51                                     HCl Dur. 0.46      0.44      0.45    0.61                                     CTE      36.3      36.6      36.6    37.6                                     ______________________________________                                    

                  TABLE IA                                                        ______________________________________                                                 1    2           3      4                                            ______________________________________                                        SiO.sub.2  68.6   68.2        68.3 67.3                                       Al.sub.2 O.sub.3                                                                         11.1   11.4        11.3 11.4                                       B.sub.2 O.sub.3                                                                          8.25   8.31        8.22 8.5                                        MgO        1.27   1.28        1.32 1.35                                       CaO        5.08   5.11        5.11 5.32                                       SrO        1.26   1.27        1.27 1.28                                       BaO        3.89   3.9         3.91 4.39                                       SnO.sub.2  0.03   0.04        0.04 --                                         Sb.sub.2 o.sub.3                                                                         0.44   0.44        0.44 --                                         As.sub.2 O.sub.3                   0.4                                        ______________________________________                                    

These examples, which are meant to be illustrative and not limiting,demonstrate that aluminoborosilicate glasses such as those fallingwithin the compositional ranges described above can be made usingdowndraw manufacturing processes.

In a preferred embodiment of the invention, the glass is formed in amanufacturing system which employs platinum, molybdenum, palladium,rhodium or an alloy thereof in contacting relationship with the glass,and the partial vapor pressure of hydrogen outside this portion of themanufacturing ssytem relative to the partial vapor pressure of hydrogenin the glass or inside that manufacturing vessel. The partial pressureof hydrogen outside the vessel can be controlled, for example, byenclosing part of the vessel in an enclosure, and varying the partialpressure of hydrogen, or the dew point, inside the enclosure as desired.By so controlling the relative partial pressures of hydrogen insideversus outside the platinum or molybdenum containing portion of theglass manufacturing system, we can control, and if desired, reduce theamount of surface blisters which were heretofore problematic in suchglass manufacturing systems which employed platinum or molybdenum. Thepartial vapor pressure of hydrogen inside and outside the system can becontrolled, for example, by controlling the partial vapor pressure ofwater inside and outside the system. The desired relative partialpressures inside versus outside the forming vessels depends upon whetherthe forming vessel contains platinum or molybdenum (or palladium orrhodium) as a glass contacting material.

For example, platinum is desirable for use in glass forming vesselsprimarily because of its inert properties. However, platinum enableshydrogen migration to occur from the glass melt through the platinum,thereby creating an oxygen rich layer at the glass/platinum interfacewhich results in surface blisters. Consequently, for the platinum glassmanufacturing vessels, it is most desirable to maintain the relativeinside and outside partial pressures of hydrogen to be substantiallyequal, so that no migration of hydrogen, either in or out of the glassmanufacturing vessel, occurs. However, if any migration is to occur, itis more desirable that it occur from the outside in, and thus in anotherembodiment the partial pressure of hydrogen outside the platinum ormolybdenum manufacturing vessel is maintained at a higher level then ispresent inside the vessel.

On the other hand, molybdenum acts as a reducing material toward oxidemelts. Therefore it is desirable to maintain a partial pressure ofhydrogen outside the molybdenum containing forming vessel which is lowerthan that inside the forming vessel, to reduce the amount of blistersformed as a result of reduction of the glass constituents (e.g.formation of SO₂ bubbles as a result of reduction of dissolved SO₃).

In another preferred embodiment, a measurement device is used to measurethe relative partial pressures of hydrogen inside versus outside themanufacturing vessel, and the humidity or dew point outside the vesselis then controlled accordingly. A preferred such measurement apparatusis shown in FIG. 1. Platinum vessel 10 includes platinum walls 12through which molten glass 13 is flowing. The platinum walls 12 ofvessel 10 can have any shape (e.g. round or rectangular incross-section), and the direction of flow of the molten glass throughvessel 10 is not important. A platinum flag electrode 14 is immersedinto the molten glass. By platinum flag, it is meant a flat sheet ofplatinum, so that the both sides of the platinum contact the moltenglass, and therefore the flag does not experience hydrogen permeation.Also immersed into the molten glass 13 is a platinum tube 20, theinterior of which is in contact with the atmosphere outside platinumvessel 10. Both flag electrode 14 and the platinum tube 20 are isolatedfrom the platinum manufacturing vessel 10 via an insulating material 24.The flag electrode 14 and platinum tube 20 are then connected asillustrated in FIG. 1. Controller 15 is used to adjust the voltage fromvariable d.c. power source 16 necessary to maintain the target potentialbetween electrodes 14 and 20. The current necessary to sustain thisvoltage is then read from ammeter 17 as an indicator of the flow ofhydrogen through the platinum wall 20, and thus platinum wall 12 aswell. For example, an increase in current indicates a net decrease inthe rate of hydrogen migration out of the glass and into the atmosphereoutside the Pt system. Conversely, a decrease in current would indicatea net increase in the rate of hydrogen migration out of the glass andinto the atmosphere.

The apparatus illustrated in FIG. 1 is enclosed by enclosure 30 (shownschematically), which enables the control of the partial pressure ofhydrogen surrounding the platinum vessel 10. Thus, if the measurementsystem described above indicates a change from the target potential, thehumidity inside enclosure 30 can be adjusted to correct for this change.Other variations within the scope of the claimed invention will beapparent to those skilled in the art. For example, the potential betweenflag 14 and platinum tube 20 could be monitored simply by a voltageregulator, and a signal produced relative to the voltage measured, thesignal then being sent to a control device capable of increasing ordecreasing the humidity or dew point in the enclosure in response to thesignal. In addition, while in FIG. 1, only a portion of the vessel 10 isenclosed, in a preferred embodiment the entire portion of themanufacturing process employing platinum vessels is enclosed. Clearly, asimilar control system can be devised if the partial pressure ofhydrogen is varied directly.

Such control of the relative partial pressures of hydrogen, as well asthe measuring device described above, are explained in more detail inU.S. patent application Ser. No. 08/736,848, filed simultaneouslyherewith, the specification of which is hereby incorporated byreference.

Although the invention has been described in detail for the purpose ofillustration, it is understood that such detail is solely for thatpurpose and variations can be made therein by those skilled in the artwithout departing from the spirit and scope of the invention which isdefined by the following claims.

What is claimed is:
 1. A method of making a silicate glasscomprising:melting and downdraw forming a silicate sheet glass andselecting batch constituents of said silicate glass so that theresultant glass contains less than 0.2 mole percent arsenic expressed asAs₂ O₃, and the β-OH of said glass is below about 0.5/mm.
 2. The methodof claim 1, wherein said downdraw glass manufacturing process is a sheetforming downdraw process.
 3. The method of claim 2, wherein the batchconstituents and process control variables in said melting step areadjusted so that the β-OH is below 0.4/mm.
 4. The method of claim 2,wherein the batch constituents and process control variables in saidmelting step are adjusted so that the β-OH is below 0.35/mm.
 5. Themethod of claim 2, wherein said melting step comprises forming saidsheet using a downdraw process and said glass is contacted with platinumduring said melting or forming steps.
 6. The method of claim 5, whereinsaid downdraw process in said melting step is a fusion process.
 7. Themethod of claim 2, wherein said melting step further comprises employinga fining agent selected from the group consisting of Sb₂ O₃, CeO₂, SnO₂,Fe₂ O₃, halide containing compounds, and mixtures thereof.
 8. The methodof claim 3, wherein the batch constituents and process control variablesare adjusted so that said glass comprises less than 0.1 mole percent AS₂O₃.
 9. The method of claim 5, wherein the batch constituents and processcontrol variables are adjusted so that said glass in said melting stepis essentially free of arsenic.
 10. The method of claim 2, wherein saidmelting step comprises employing antimony containing material in anamount which results in the resultant glass having between 0.02 to 1mole percent Sb₂ O₃.
 11. The method of claim 7, wherein said meltingstep comprises employing a halide containing compound in an amountsufficient to result in about 0.1 to 2 mole percent halide in theresultant glass.
 12. The method of claim 11, wherein the halide in saidmelting step is chlorine.
 13. The method of claim 2, wherein theresultant glass comprises an aluminoborosilicate glass, expressed interms of mole percent on the oxide basis, having:

    ______________________________________                                        SiO.sub.2                                                                              60-73          MgO    0-5                                            Al.sub.2 O.sub.3                                                                        8-14          CaO    1-13                                           B.sub.2 O.sub.3                                                                         5-17          SrO    0-8                                            TiO.sub.2                                                                              0-5            BaO    0-14                                           Ta.sub.2 O.sub.5                                                                        0-5.                                                                ______________________________________                                    


14. The method of claim 13, wherein the resultant glass is essentiallyfree of alkali metal oxides and exhibits a strain point higher than 630°C., and a linear coefficient of thermal expansion over the temperaturerange 0°-300° C. between 32-46×10⁻⁷ /°C.
 15. A method of making asilicate glass comprising:melting a silicate glass mixture having amelting point greater than 1500° C., selecting batch constituents insaid melting step so that the resultant glass contains less than 0.2mole percent arsenic expressed as As₂ O₃, and the β-OH of said glass isbelow about 0.5/mm, and forming a glass from said mixture; and at somepoint during said method of making said glass is in the presence ofplatinum.
 16. The method of claim 15, wherein the batch constituents andprocess control variables are adjusted so that the β-OH is below 0.4/mm.17. The method of claim 15, wherein the batch constituents and processcontrol variables are adjusted so that the β-OH is below 0.35/mm. 18.The method of claim 15, further comprising forming said sheet using aprocess which does not employ floating of the glass sheet on a moltentin bath.
 19. The method of claim 16, further comprising forming a sheetof said glass using a process wherein glass is contacted with platinumduring the melting or forming process.
 20. The method of claim 19,wherein said process is a downdraw sheet forming process.
 21. The methodof claim 18, wherein the batch constituents and process controlvariables are adjusted so that said glass comprises less than 0.1 molepercent AS₂ O₃.
 22. The method of claim 19, wherein the batchconstituents and process control variables are adjusted so that saidglass in said melting step is essentially free of arsenic.
 23. Themethod of claim 18, wherein said glass in said melting step comprises analuminoborosilicate glass, expressed in terms of mole percent on theoxide basis, having:

    ______________________________________                                        SiO.sub.2                                                                              60-73          MgO    0-5                                            Al.sub.2 O.sub.3                                                                        8-14          CaO    1-13                                           B.sub.2 O.sub.3                                                                         5-17          SrO    0-8                                            TiO.sub.2                                                                              0-5            BaO    0-14                                           Ta.sub.2 O.sub.5                                                                        0-5.                                                                ______________________________________                                    


24. The method of claim 23, wherein said glass in said melting step isessentially free of alkali metal oxides and exhibits a strain pointhigher than 630° C., and a linear coefficient of thermal expansion overthe temperature range 0°-300° C. between 32-46×10⁻⁷ /°C.
 25. The methodof claim 1, wherein said melting and forming step comprises melting andforming a mixed oxide silicate glass.
 26. The method of claim 1, whereinsaid melting and forming step comprises melting and forming a glasswhich is essentially alkali free.
 27. The method of claim 15, whereinsaid melting step comprises melting a mixed oxide silicate glass. 28.The method of claim 15, wherein said melting step comprises melting aglass which is essentially alkali free.
 29. The method of claim 1,wherein said glass sheet is essentially free of gaseous inclusions. 30.The method of claim 15, wherein said sheet is essentially bubble-free.